In xerographic printing (also called electrophotographic printing), a latent image is formed on a charged photoreceptor, usually by raster sweeping a modulated laser beam across the photoreceptor. The latent image is then used to create a permanent image by transferring and fusing toner that was electrostatically attracted to the latent image onto a recording medium, usually plain paper. While xerographic printing has been successful, problems arise when attempting to print at high speed. One set of problems relates to the sweeping of the laser beam across the photoreceptor.
As printing speed increases, it becomes more and more difficult to sweep the laser beam as fast as is required. While other methods are known, the most common method of sweeping the laser beam is to deflect it from a rotating mirror. One method of increasing the sweep speed is to rotate the mirror faster. While this method can increase the speed of the raster sweep, to rotate the mirror extremely fast requires an expensive drive motor and bearings.
Other techniques to increase the effective raster sweep speed are to 1) sweep the beam using a multifaceted, rotating polygon mirror having a set of related optics, or 2) sweep several beams simultaneously. Rotating polygon mirrors and their related optics are so common that they are generically referred to as ROSs (Raster Output Scanners), while printers that sweep several beams simultaneously are referred to as multispot printers. Both techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura.
The sweep rate problem becomes even more apparent when printing multiple colors, such as in a full color print, at high speed. This is because a xerographic printer that prints in two or more colors requires a separate latent image for each color printed, hereinafter called a system color. While a two color printer requires only two latent images, a full color printer typically uses the three primary colors of cyan, magenta, yellow, plus black, and thus four latent images are required. Color prints are currently produced by sequentially transferring and fusing overlapped images of each system color onto a single recording medium that is passed multiple times, once for each system color, through the printer. Such printers are called multiple pass printers. Conceptually, one can imprint multiple colors on a recording medium that is passed through the system only once by using a sequence of multiple xerographic stations, one for each system color. Such a printer, called hereinafter a multistation printer, would have a greater output then a multipass printer operating at the same raster sweep speed. However, the introduction of multistation printers has been delayed by 1) cost problems, at least partially related to the cost of multiple xerographic stations and the associated ROSs, and 2) image quality problems, at least partially related to the difficulty of producing similar spots on each photoreceptor and subsequently registering (overlapping) the latent images on the photoreceptors.
Proposed prior art multistation printers have usually included individual ROSs (each comprised of separate polygon mirrors, lenses, and related optical components) for each station. For example, U.S. Pat. Nos. 4,847,642 and 4,903,067 to Murayama et al. involve such systems. Problems with these systems include the high cost of producing nearly identical multiple ROSs and the difficulty of registering the system colors.
A partial solution to the problems of multistation xerographic systems with individual ROSs is disclosed in U.S. Pat. No. 4,591,903 to Kawamura et al. The '903 patent, particularly with regards to FIG. 6, discusses a recording apparatus (printer) having multiple recording stations and multiple lens systems, but only one polygon mirror. With only one polygon mirror and associated drive motor, the cost of the system is reduced. However, differences in the lenses and mirror surfaces still could cause problems with color registration.
Another approach to overcoming the problems of multistation printers having individual ROSs is disclosed in U.S. Pat. No. 4,962,312 to Matuura et al. The '312 patent illustrates spatially overlapping a plurality of beams using an optical beam combiner, deflecting the overlapped beams using a single polygon mirror, separating the deflected beams using an optical filter (and polarizers if more than two beams are used), and directing the separated beams onto associated photoreceptors. The advantage of overlapping the laser beams is a significant cost reduction since the ROS is shared.
However, an actual embodiment of the '312 apparatus would be rather complicated and expensive, especially if four system colors are to be printed. The use of optical beam combiners to overlap beams so that they have similar optical axes would be difficult, expensive, and time consuming. Obtaining similar sized spots on each photoreceptor would be also be difficult as it would be difficult to establish the same optical path lengths for each beam. Finally, it would also be difficult to ensure that the latent images on the photoreceptors are registered. Each of these problems is at least partially related to the relative positions of the laser sources.
What is needed is a raster output scanner (ROS) system suitable for deflecting multiple laser beams in a multistation printer. The ROS should deflect multiple laser beams having substantially parallel optical axes by rotating a common mirror surface area, separate the deflected laser beams, and direct each beam onto its respective photoreceptor such that similarly dimensioned spots, each substantially in registration with spots on the other photoreceptors, are obtained.